KA-CHING! John Giuffrida, who collects a $75,000 FDNY disability pension for lung disease, is an MMA fighter. The Post has also exposed (pictured) John McLaughlin, a marathon runner getting a huge lung-disease FDNY disability. Photo: www.mgfightgear.com

That’s one knockout of a retirement.

A former city firefighter who retired with a nearly $75,000-a-year tax-free disability pension seven years ago for alleged lung problems is now a mixed-martial-arts fighter, The Post has learned.

John Giuffrida, 42, has spent the last two years wiping the mats with opponents in brutal grappling and Thai-kickboxing matches despite his career-ending diagnoses of lung disease and post-traumatic stress disorder.

“He’s scamming the city and slapping all these widows and widowers in the face,” a source close to Giuffrida said, adding that he got divorced and took up hiking and scuba diving in his retirement along with martial arts.

“All of a sudden he was cured,” the source fumed.

Giuffrida claims that he was forced to retire by a Fire Department medical board after 12 years of service — collecting a three-quarters-pay pension that the FDNY said comes out to $74,624 a year.

Among the first-responders on 9/11, Giuffrida says he spent 30 days — including 30 hours nonstop — doing the grim and toxic recovery work at Ground Zero.

The next year, FDNY docs determined he was suffering from asthma and other lung ailments and placed him on light duty. By 2003, he was out with his tax-free disability pension despite his attempts to stay on, he said.

“I have nothing to hide,” Giuffrida told The Post from his Shark River, NJ, home. “The conditions that precluded me from being a firefighter in no way preclude me living an active life in order to preserve my health.”

Giuffrida is the latest firefighter found drawing a disability pension while performing feats of strength and endurance.

The Post has reported that retired FDNY Lt. John McLaughlin competes in marathons and triathlons while collecting an $86,000 disability pension for lung disease.

The city’s pension costs are expected to reach $7.6 billion this year — up from $1.4 billion when Mayor Bloomberg took office in 2002. Next year, it’s expected to hit $8.7 billion.

Giuffrida defended his post-retirement athletics and the punishing regimen that’s needed to prepare for the almost no-holds-barred grappling matches.

“It is completely different than running into a building that is on fire with a smoke condition and toxins in the air,” Giuffrida said.

The 6-foot, 197-pound Giuffrida took the Bronze in the 2010 North American Grappling Association World Championship this past April in Newark.

His banner year in the mixed-martial-arts world was in 2008, according to the Web site Modern Gladiator Fight Gear, the company that sponsors his amateur fight team.

That year, the “disabled” man won two different North American Grappling Association championships as well as a local championship.

Neighbors sang Giuffrida’s praises.

“He’s a super-great guy,” said Laura Welsh, who recalled how he once chased a prowler from the block.

Giuffrida is still suffering — either from his punishing bouts or lingering effects of 9/11, Welsh said.